Grigor Dimitrov

The 26-year-old Bulgarian heads to Melbourne a dizzy No 3 in the world rankings behind only Rafael Nadal and Roger Federer.

Nicknamed "Baby Fed" early in his career for his Federer-style game, Dimitrov has struggled to live up to the comparison and was as low as 40 in the world in mid-2016. But he bounced back to enjoy the best season of his career in 2017, winning four titles, including his first Masters crown in Cincinnati.

Dimitrov has the task this year of defending his rankings points and backing up a top-five season. He has never been beyond the semifinals of a Grand Slam, reaching the last four at Wimbledon in 2014 and repeating the feat in Australia last year.

Alexander Zverev

The giant German, seen as a trailblazer for tennis' widely touted Next Generation, heads into the year's first Grand Slam ranked fourth, and is coming off five titles last year.

Zverev, 20, is one of just four men to beat Federer in 2017 when he defeated the Swiss great in the Montreal final. But Zverev has never made it past the fourth round of a Grand Slam, and he's reached that stage only once, at Wimbledon in 2017.

He generates enormous power off his 6-foot-6 frame and pundits say as he bulks up and gains more experience in five-set matches, success at all tournaments, including Grand Slams, should follow.

Dominic Thiem

The 23-year-old Austrian finished the 2017 season with a title on clay in Rio de Janeiro and a year-end No 5 ranking. Thiem claimed his first win over Novak Djokovic at the French Open quarterfinals on the way to reaching the semifinals at Roland Garros for the second straight year.

His boyhood idols were countrymen Stefan Koubek and Jurgen Melzer, while his favorite shot is the forehand and he prefers to play on clay. Thiem, who pulled out of the warmup Qatar Open ahead of his semifinal with flu, finished 2017 by making his second appearance at the ATP Finals in London.

Nick Kyrgios

The volatile Aussie, whose potential has often been betrayed by his temperament, brought last season to an early end when he surrendered to a longstanding hip injury. He started 2018 in top form, winning the Brisbane International on Sunday, beating Dimitrov along the way.

Kyrgios is aiming for a fresh assault on his home Grand Slam, where he threw away a two-set lead to lose to Andreas Seppi in the second round last year. The 22-year-old fan favorite, ranked No 17, has the talent to beat the best but so often undermines his brilliance with petulance directed at umpires or fans. Has two career wins over Rafael Nadal. Remains a dangerous opponent if his hip and attention span hold up.

Andrey Rublev

The 20-year-old Russian captured his first ATP World Tour title as a lucky loser at Umag before he became the youngest US Open quarterfinalist since Andy Roddick by beating Grigor Dimitrov and David Goffin last year before losing to Nadal. Now ranked a career-high No 32, the run in New York solidified Rublev's status as one of the top 'Next Gen' ATP players in the world.